A 4-wheel drive (4WD) vehicle exerts a driving force on all four wheels. Thus, the engine distributes power to both front and rear axles to maintain traction under all conditions. 4WD systems are classified as either part time or full time 4WD systems.
In a part time 4WD system, the vehicle operates in a 2-weel driving mode during normal conditions. However, upon encountering rough terrain or adverse weather conditions, a 4-wheel drive mode is engaged by using a 4WD transfer case to transfer a strong driving force or torque to all 4 wheels. In a full time 4WD system, all four wheels exert the driving force. Therefore, a full time 4WD system ensures stable traction both during normal conditions and adverse weather conditions, and therefore, many expensive vehicles come equipped with a 4WD transfer case.
Recently-built full time 4WD systems include an AUTO mode and a LOW mode. In the AUTO mode, the revolution differences between the front wheels and the rear wheels are electronically controlled by a multi-plate clutch, which engages the 2WD to 4WD, and vice versa. When maximum driving force or torque is required to maneuver the vehicle out of rough terrain or pull another vehicle, the system is shifted to the LOW mode.
Conventional transfer cases, however, have complicated mechanisms for shifting from the AUTO mode to the LOW mode and vice versa. Further, the number of the required components is large, and the manufacturing cost is high.